The present invention relates to shock sensors in general and shock sensors employing a reed switch in particular.
Reed switches have long been used in shock sensors because of their high reliability, low cost, and relative immunity to electromagnetic interference. It is this resistance to electromagnetic interference, along with other factors, to which they owe their continued utility in the face of the widespread availability of solid-state shock sensors. Reed switch based shock sensors are widely used in combination with solid-state shock sensors. The reed switch based shock sensor provides assurance that an actual crash is taking place, while the solid-state shock sensor provides characterization of the magnitude and direction of the sensed shock. However, the advantages of reed switch based shock sensorsxe2x80x94macro scale and hence resistance to electromagnetic interferencexe2x80x94are also their principal liability in as much as the physical size of the shock sensor takes up considerable real estate on a circuit board. A typical reed switch based shock sensor consumes perhaps 400 square mm of real estate.
What is needed is reed switch based shock sensor which is substantially reduced in scale.
The shock sensor of this invention employs a reed switch contained within a plastic housing. A shock sensing magnet is biased to one side of the housing by a spring. The shock sensing magnet is cylindrical and has a cylindrical bore and the reed switch is positioned within the cylindrical bore. The interior cylindrical surface defined by the magnet cylindrical bore rides in sliding engagement on the reed switch glass capsule. The magnet functions as a shock sensing mass, and is shaped to increase the reed switch dwell time. The reed switch has two opposed ferromagnetic leads which extend axially concentric with the cylindrical bore and through opposite sides of a plastic housing. The leads are bent to extend downwardly along the sides of the plastic housing and then are bent horizontally so as to be parallel to the sides of the housing and to a circuit board on which the shock sensor is mounted. The housing is formed of two parts which are ultrasonically welded together. This welding hermetically seals the housing about the reed switch leads. A strip of mu-metal wraps three sides of the plastic housing and extends partly beneath the housing so that the housing may be attached to the circuit board by soldering to the circuit board tabs of mu-metal which extend beneath the housing. The magnet and the housing are constructed from plastics which can withstand the momentary high temperature associated with a re-flow solder process.
It is an feature of the present invention to provide a reed switch based shock sensor which uses less real estate on a circuit board.
It is a further feature of the present invention to provide a reed switch based shock sensor which incorporates magnetic shielding.
It is another feature of the present invention to provide a reed switch based shock sensor suitable for surface mounting to a circuit board.
It is a yet further feature of the present invention to provide a reed switch based shock sensor suitable for mounting to a circuit board with the re-flow solder process.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.